Sea urchins, or simply urchins, are globular echinoderms that belong to the class Echinoidea. These animals begin their life as bilaterally symmetrical larvae, but most species develop into adults with a five-fold symmetry. Their bodies are encased in a hard shell called the test, which contains numerous spines and pores that allow their tube feet to extend outwards.
Currently, about 950 species of sea urchins are found across the tropics to the polar regions, typically settling on the seabed. They usually crawl on their tube feet and search for algae, seaweed, and other plant material, though they may occasionally feed on slow-moving invertebrates. Some species of these sea urchins are edible and consumed as a delicacy in countries like Japan, Vietnam, and South Korea. Since the 19th century, they have been widely used as model organisms in developmental studies for the ease of studying their embryos.
Most sea urchins measure between 3 and 10 cm (1 and 4 in) across their test, though the largest species, the red sea urchin (Mesocentrotus franciscanus), grows up to 18 cm (7 in) in test diameter, with a spine length of 8 cm (3 in). In contrast, the smallest species, Echinocyamus scaber, has a test diameter of barely 5.5 cm (0.22 in).
These animals typically possess rigid, spherical bodies covered in numerous spines. While their larvae exhibit bilateral symmetry, the adults develop five-fold symmetry (pentamerism), like most echinoderms. In most species, the mouth is positioned on the lower surface (oral side), and the anus is located on the upper surface (aboral side).
In some groups, such as sand dollars (order Clypeasteroida), the adults possess a secondarily bilateral symmetry, which stems from an originally pentamerous symmetry, resulting in flat, oval-shaped body forms. Their upper (dorsal) surface is slightly domed, while the lower (ventral) surface is flat.
A sea urchin’s body is covered by a rigid shell or test comprising fused plates of calcium carbonate. It is formed internally beneath a thin epidermis, followed by a dermis and a thin layer of muscle and skin. Hence, despite being a covering, the test is also often referred to as the endoskeleton of these animals.
The test typically has five longitudinal columns, comprising five ambulacral grooves alternately separated by five wider interambulacral grooves. Each column is composed of two rows of plates, the ambulacral and interambulacral plates. While the ambulacral plates feature tiny holes (or pores) that allow the tube feet to extend, the interambulacral plates lack them. The interambulacral plates, and, to a lesser extent, the ambulacral plates, too, bear small tubercles that serve as articulation points for spines. These spines, often venomous, serve as their defense by puncturing the bodies of their enemies.
In most sea urchins, the spines are arranged in vertical rows along the interambulacral and ambulacral plates. Most plates typically have a long primary spine and multiple smaller secondary spines. The spines are usually cylindrical and hollow, with the shortest located at the poles and the longest at the equator. Nestled among the spines are tiny claw-shaped structures with jaws known as pedicellariae. These structures play an important role in defense, capturing food, and removing unwanted substances from the surface of their bodies.
As echinoderms, sea urchins have an elaborate water vascular system that leads downwards from a calcareous opening called the madreporite. This opening leads to a longitudinal stone canal, followed by a ring canal, which surrounds the esophagus.
Radial canals spread out from the ring canal into each ambulacral groove, leading to lateral canals that end in bulbous ampullae. Each ampulla is connected to a single tube foot that extends outwards through the pores on the test.
This system works under hydraulic pressure and aids in both locomotion and feeding.
Sea urchins lack a true circulatory system with blood and a true heart. Instead, they possess a coelomic system comprising a body cavity (coelom) filled with coelomic fluid. This fluid comprises specialized cells called phagocytic coelomocytes, which help transport nutrients, gases, and waste products. These wastes are further expelled through the gills and tube feet. They also have a hemal system comprising an intricate network of vessels in the mesenteries around the gut. These vessels most likely also facilitate the distribution of nutrients throughout the body of the animal[1].
The mouth is composed of five teeth or plates made of calcium carbonate. Within these plates lies a tongue-like structure, which helps procure prey. The mouth is surrounded by an area of soft, membranous tissue called the peristome, which includes multiple tiny bony pieces, five pairs of modified tube feet, and five pairs of gills (when present).
The jaws comprise five strong, triangular plates known as pyramids. Each plate bears a toothband on the ventral surface, containing a hard tooth pointing towards the center of the mouth. There are specialized muscles that control this jaw apparatus (also called Aristotle’s lantern), thereby controlling the action of the teeth.
The pharynx leads to the esophagus, which loops down the mouth to connect to the small intestine and a single caecum. The small intestine loops around the inside of the test and joins the large intestine, which, in turn, loops around in the opposite direction. The large intestine leads to the rectum, followed by an anus. An additional tube, the siphon, runs along the intestine and likely aids in water resorption.
In the aboral pole of a sea urchin lies a membrane called the periproct, which surrounds the anus. This membrane is composed of numerous hard plates, five of which comprise the genital plates and contain the genital openings or gonopores.
Sea urchins do not have lungs or a typical gill system as found in vertebrates. Most species primarily respire through their tube feet, across which gaseous exchange occurs with the surroundings. In addition to respiration through their tube feet, some species, like the purple sea urchin, possess five pairs of peristomial gills around the mouth, which play a secondary role in respiration[2].
They have a simple nervous system lacking a true brain. Instead, they have a large nerve ring surrounding the mouth. Five nerves radiate from this ring into the five ambulacral grooves, which further branch into finer nerves that innervate the tube feet and spines.
Though they lack eyes or eye spots, sea urchins have numerous sensory cells embedded in the epithelium, particularly in the spines, tube feet, and around the mouth. Hence, their entire body could be referred to as a single compound eye.
A few species, like the purple sea urchin, possess globular organs called spheridia near the base of the spines. These organs most likely help in maintaining balance, though their exact function remains unconfirmed[3].
Regular sea urchins possess five gonads lying underneath the interambulacral areas of the test, whereas non-pentamerous (irregular) forms have fewer (typically four) gonads due to their bilateral symmetry. Heart urchins of the order Spatangoida have two or three gonads.
Each gonad has its own duct that drains into an opening called the gonopore, which lies in one of the genital plates around the anus. A set of muscles above the gonads contract to squeeze their gametes through the gonopore.
Together with sea cucumbers (class Holothuroidea), sea urchins (class Echinoidea) form the subphylum Echinozoa. The name of the class, Echinoidea, is obtained from the Greek word ekhinos, meaning hedgehog, which refers to the hedgehog-like, spiny appearance of its members. Similarly, the terms ‘sea urchin’ or ‘urchin’ originate from the Old French word herichun and the Latin ericius, both meaning hedgehog. As a result, they were also historically referred to as sea hedgehogs.
Around 950 species of sea urchins are currently divided into 2 subclasses and 13 orders.
In an alternate taxonomy, a separate infraclass called Irregularia is recognized within the subclass Euechinoidea. As the name suggests, this infraclass includes all sea urchins with non-pentamerous body shape, such as sand dollars and heart urchins.
Since these echinoderms are equipped with a hard test, they are easily preserved and have a rich fossil record. The earliest fossil specimens of sea urchins date back to around 465 million years ago in the Middle Ordovician Period.
Fossils from the Palaeozoic Era, particularly from the Devonian and Carboniferous Periods, are found only in parts, mostly comprising isolated spines and scattered plates. Specimens from the Ordovician and Silurian Periods unearthed from the shallow-water limestones in Estonia suggest that early sea urchins with thin tests probably lived in relatively quieter waters than modern forms.
Very few species (only six, as of now) are known from the Permian Period, suggesting this group almost went extinct at the end of the Paleozoic Era. However, two groups survived into the Triassic Period: ancestors of modern pencil urchins (order Cidaroida in the subclass Perischoechinoidea) and modern euechinoids (subclass Euechinoidea). By the time they reached the Upper Triassic Period, sea urchins started to increase in number once again. In the Jurassic and Cretaceous Periods, the euechinoids diversified, giving rise to multiple lineages, including the Atelostomata, the group of the first irregular sea urchins. Eventually, in the Paleogene and Neogene Periods, sand dollars (order Clypeasteroida) originated. Members of this group had flattened tests and tiny spines and gradually adapted to loose sand in shallow water.
Although sea urchins inhabit every ocean, they are most commonly found along temperate and tropical coasts.
Most species are found on seabeds (benthic), ranging from the intertidal zone to thousands of meters below the sea. Some cidaroids, particularly those in the family Echinothuriidae and the genus Dermechinus, inhabit the abyssal zone at depths of 13,000 to 20,000 ft (4000 to 6000 m). Moreover, some members of the family Pourtalesiidae are among the deepest-living sea urchins, typically found in the hadal zone, reaching depths of as much as 22,470 ft (6,850 m) in the Sunda Trench.
The population densities of sea urchins vary according to the habitat type. For example, they have the maximum density in barren areas compared to those rich in kelps. Some species, like the shingle urchin (Colobocentrotus atratus), are resistant to wave action and are one of the few sea urchins that can survive periodic exposure to air during low tides[4].
In the larval stage, sea urchins typically feed on phytoplankton and particulate organic matter. On reaching adulthood, they adopt a primarily herbivorous diet based on macroalgae, like kelp, red algae, and green algae. However, when their primary resources are scarce, sea urchins feed on invertebrates, such as mussels, conches, polychaetes, sponges, and even other echinoderms like sea cucumbers, brittle stars, and crinoids.
These animals typically move by crawling on their tube feet, relying on hydraulic pressure generated by the water vascular system. Occasionally, their spines assist by pushing the body along the substrate or lifting it off the surface. Some species, like the purple sea urchin, additionally use their jaws to scrape the sediment.
When food is abundant, species like red sea urchins make minimal movement, covering about 7.5 cm per day. However, if resources are scarce on a particular day, this species can travel up to 50 cm a day in search of food.
The lifespan of these animals varies considerably with species. For instance, red sea urchins survive over 100 years[5], while purple sea urchins live for about 20 years on average. According to the Animal Ageing and Longevity Database, some red sea urchins in Canada have been found to survive up to 200 years. In contrast, the European edible sea urchin has an average lifespan of only 6 to 10 years[6].
Most echinoderms are dioecious, meaning they have separate male and female sexes. They reproduce sexually, with both males and females releasing their gametes (sperm and eggs) into the surrounding water through their gonopores (broadcast spawning).
In most cases, the unfertilized eggs float freely in the water, though some species may hold them in their spines. As the egg meets the free-floating sperm, they fuse and fertilize externally, giving rise to the single-celled zygote. Gradually, the zygote undergoes repeated divisions to form a multicellular embryo.
The cells of the embryo start to undergo multiple divisions and rearrangements before giving rise to the larva.
In approximately 12 hours, the embryo undergoes ten cycles of cell division, forming a ball-like structure called the blastula. This stage includes a blastocoel, a fluid-filled cavity, and a single layer of epithelial cells surrounding the blastocoel.
The blastula divides further, undergoing gastrulation to form the gastrula. The development of the gastrula includes:
At the end of gastrulation:
This stage, known as the echinopluteus, is characterized by 12 long arms (in most species) lined with bands of cilia for capturing food. However, in some species, like Heliocidaris erythrogramma, the blastula is so rich in yolk that the larvae do not need to feed.
The larva undergoes the following sequential stages of development:
The fully developed echinopluteus sinks to the bottom, settles on the substrate, and metamorphoses into the juvenile in as little as one hour in some species, like the common heart urchin[7]. The juvenile then develops tube feet, spines, and a protective test.
As juveniles grow, they gradually develop the characteristic pentaradial symmetry and transform into fully mature adults over time. These adults attain sexual maturity in a few years, depending on the species.
They are preyed upon by crustaceans, such as crabs and spiny lobsters, cephalopods, like octopuses, and several fish, including wolf eels, pufferfish, triggerfish, and wrasses. Sea otters also feed on these echinoderms. Some species of sharks, like the horn shark (Heterodontus francisci), have been found to feed on sea urchins[8].
Since sea urchins primarily graze on algae and kelps, they play an important role in the marine ecosystem by controlling their excess growth. By reducing algal dominance, these echinoderms enable the growth of corals and thus stabilize the reef community.
Only one sea urchin species, the European edible sea urchin (Echinus esculentus), is currently listed under the Near Threatened (NT) category of the IUCN Red List of Threatened Species.
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